The National Marrow Donor Program (NMDP) was established in 1987 to (1) create a registry of volunteer, tissue-typed, unrelated bone marrow donors and (2) facilitate matched unrelated donor marrow transplants through a coordinated circuit of donor, collection, and transplant centers. As of August 31, 2004, 5.5 million donors were participating in the registry and greater than 19,549 unrelated stem cell transplants had been facilitated. Peripheral blood stem cell (PBSC) components, harvested by apheresis of filgrastim-stimulated donors, provide larger numbers of progenitor cells which engraft more rapidly than marrow-derived stem cells, and are currently exceeding the use of marrow in both related and unrelated donor settings. The NIH Marrow Donor Center, one of the largest hospital-based donor centers participating in the NMDP network, with 65,600 donors on its registry, is participating in a nationwide NMDP protocol for the acquisition of filgrastim-stimulated PBSCs by apheresis of unrelated donors. The objectives of these studies are (1) to monitor the safety of filgrastim administration in healthy volunteer donors, (2) to compare the adverse effects of bone marrow versus PBSC donation, and (3) to monitor the outcome of matched unrelated-donor PBSC transplants, including time to engraftment, incidence of GVHD, and disease free and overall survival. As of August 24, 2004, 100 NIH donors had undergone 119 apheresis procedures to collect PBSC's for unrelated NMDP recipients. 81 of 100 (81%) required only a single apheresis procedure to collect an adequate cell dose for transplant. 19 of 100 had a poor CD34 mobilization response to filgrastim, and needed two consecutive apheresis procedures to collect an adequate cell dose. Five of the 100 donors, all female, required a central line. All donors experienced G-CSF-induced fatigue, insomnia, bone pain, or headache, although in only 6% were these effects considered severe. Peak mean leukocyte counts after filgrastim were 40,900/uL, and postapheresis thrombocytopenia (less than 100,000/uL) occurred in 13 of 100 donors (13 percent), nine of whom underwent two procedures. The mean time to complete recovery from PBSC donation was 1 week, compared with 3 weeks for marrow harvest. Ten of 15 donors who had donated both marrow and PBSC preferred G-CSF-stimulated apheresis donations to marrow harvest due to the lack of need for anesthesia and hospitalization; discomfort of the two procedures was considered equivalent. Analysis of NMDP recipient outcomes shows that PBSC transplants are associated with reduced times to engraftment and improved acute transplant-related morbidity compared with marrow transplants. However, GVHD incidence and severity are increased with PBSC versus marrow grafts, so that overall survival at one year is not different among the two types of unrelated transplants. Administrative and statistical support for this study is provided by the NMDP National Office. Filgrastim is provided under an IND agreement with Amgen (BB-IND #6821).